Method for making lithographic printing plates according to the silver salt diffusion transfer process

ABSTRACT

The present invention provides a method for making an offset printing plate according to the silver salt diffusion transfer process having improved printing properties comprising the step of developing an imaging element comprising in the order given (i) a hydrophilic base, (ii) an image receiving layer containing physical development nuclei and (iii) a photosensitive layer containing a silver halide emulsion in the presence of a 1-phenyl-5-mercapto-tetrazole, being substituted by one or two substituents from the group consisting of: 
     (i) a --COOM group with M representing hydrogen, a metal cation, or an ammonium group, 
     (ii) a --SO 3  M 1  group, M 1  representing hydrogen, a metal cation, or an ammonium group, 
     (iii) an acylamido group, 
     (iv) and a substituent containing at least one of the aforementioned substituents (i)-(iv).

FIELD OF THE INVENTION

The present invention relates to a method for making improved lithographic printing plates according to the silver salt diffusion transfer process.

BACKGROUND OF THE INVENTION

The principles of the silver complex diffusion transfer reversal process, hereinafter called DTR-process, have been described e.g. in U.S. Pat. No. 2,352,014 and in the book "Photographic Silver Halide Diffusion Processes" by Andre Rott and Edith Weyde--The Focal Press--London and New York, (1972).

In the DTR-process non-developed silver halide of an information-wise exposed photographic silver halide emulsion layer material is transformed with a so-called silver halide solvent into soluble silver complex compounds which are allowed to diffuse into an image receiving element and are reduced therein with a developing agent, generally in the presence of physical development nuclei, to form a silver image having reversed image density values ("DTR-image") with respect to the black silver image obtained in the exposed areas of the photographic material.

A DTR-image bearing material can be used as a planographic printing plate wherein the DTR-silver image areas form the water-repellant ink-receptive areas on a water-receptive ink-repellant background.

The DTR-image can be formed in the image receiving layer of a sheet or web material which is a separate element with respect to the photographic silver halide emulsion material (a so-called two-sheet DTR element) or in the image receiving layer of a so-called single-support-element, also called mono-sheet element, which contains at least one photographic silver halide emulsion layer integral with an image receiving layer in waterpermeable relationship therewith. It is the latter mono-sheet version which is preferred for the preparation of offset printing plates by the DTR method.

Two types of the mono-sheet DTR offset printing plate exist. According to a first type disclosed in e.g. U.S. Pat. No. 4,722,535 and GB- 1,241,661 a support is provided in the order given with a silver halide emulsion layer and a layer containing physical development nuclei serving as the image-receiving layer. After information-wise exposure and development the imaged element is used as a printing plate without the removal of the emulsion layer.

According to a second type of mono-sheet DTR offset printing plate a hydrophilic support, mostly anodized aluminum, is provided in the order given with a layer of physical development nuclei and a silver halide emulsion layer. After information-wise exposure and development the imaged element is treated to remove the emulsion layer so that a support carrying a silver image is left which is used as a printing plate. Such type of lithographic printing plate is disclosed e.g. in U.S. Pat. No. 3,511,656.

As for other printing plates it is required that the offset printing plates belonging to the second type of mono-sheet DTR offset printing plates have good printing properties: a high contrast, a high printing endurance, good ink acceptance in the printing areas, no ink acceptance in the non-printing areas (no staining) and furthermore a low number of copies that have to be disposed off because of ink acceptance in the non-printing areas (so called toning) during start-up of the printing process. With respect to these requirements the state of the transferred silver i.e. the silver formed in the image-receiving layer plays a very important role. Parameters that are known to control the state of the transferred silver are e.g. the rate of development of the exposed silver halide (chemical development), rate of dissolution of the non-exposed silver halide by the silver halide solvent(s), rate of diffusion of the silver halide complexes, rate of development of the silver halide complexes in the image receiving layer (physical development) etc..

It has been experimentally established that the type of the photographic silver halide emulsion stabilizer used has a considerable influences on the mentioned parameters. Most of the commonly used emulsion stabilizers hamper the formation of the silver image in the receiving layer, so that an offset printing plate with inferior printing properties is obtained.

So there is a need for appropriate silver halide emulsion stabilizers in order to obtain improved lithographic printing plates according to the silver salt diffusion transfer process.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method for making offset printing plates according to the DTR-process having good printing properties i.e. a high contrast, good ink acceptance in the printing areas, no ink acceptance in the non-printing areas and a high printing endurance.

Further objects of the present invention will become clear from the description hereinafter.

According to the present invention there is provided a method for making an offset printing plate according to the silver salt diffusion transfer process comprising the steps of:

(a) image-wise exposing an imaging element comprising in the order given (i) a hydrophilic base, (ii) an image receiving layer containing physical development nuclei and (iii) a photosensitive layer containing a silver halide emulsion being in water permeable relationship with said image receiving layer,

(b) applying an aqueous alkaline solution to the imaging element in the presence of (a) developing agent(s) and (a) silver halide solvent(s) to form a silver image in said photosensitive layer and to allow unreduced silver halide or complexes formed thereof to diffuse image-wise from the photosensitive layer to said image receiving layer to produce therein a silver image,

(c) treating the imaging element to remove the layer(s) on top of the image receiving layer, thereby uncovering said silver image formed in said image receiving layer, characterized in that the development step is carried out in the presence of a 1-phenyl-5-mercapto-tetrazole being substituted by one or two substituents selected from the group consisting of:

(i) a --COOM group with M representing hydrogen, a metal cation, or an ammonium group,

(ii) a --SO₃ M¹ group, M¹ representing hydrogen, a metal cation, or an ammonium group,

(iii) an acylamido group,

(iv) and a substituent containing at least one of the aforementioned substituents (i)-(iv).

According to the present invention there is also provided an imaging element for use in the above defined method.

DETAILED DESCRIPTION OF THE INVENTION

It has been found that a lithographic printing plate prepared according to the DTR-process in the presence of a 1-phenyl-5-mercapto-tetrazole according to the present invention has an increased printing endurance, good ink acceptance at the start of the printing process and a high printing contrast. 1-phenyl-5-mercapto-tetrazole includes according to the present invention also the tautomeric thione form, wherein the hydrogen atom of the mercapto group has moved to the vicinal nitrogen atom.

The above mentioned acylamido group includes carbonamido as well as sulphonamido groups e.g. --NR--CO--R¹, --NR--SO--R¹ --NHCO--OR¹, --NH--CO--NR--R¹, --NH--CO--NR--COR¹ and --NH--CO--NR--SO₂ R¹, with R representing a substituent selected from the group consisting of hydrogen and alkyl and R¹ representing a substituent selected from the group consisting of alkyl, alkenyl, alkynyl and aryl.

Representatives of substituted 1-phenyl 5-mercapto-tetrazole compounds in accordance with the present invention are listed in the following Table 1.

                  TABLE 1                                                          ______________________________________                                         Compound       Substituents on 1-phenyl group                                  ______________________________________                                         1              3-acetamido                                                     2              3-(2-methoxyacetamido)                                          3              3-(2-methylthioacetamido)                                       4              3-(3-sulphopropionamido)                                        5              3-(2-sulphobenzamido)                                           6              3-benzsulphonamido                                              7              3-benzcarbamido                                                 8              3-benzureido                                                    9              3-benzallophanoyl                                               10             3-(2-sulphobenzamido)                                           11             4-(2-sulphobenzamido)                                           12             3-(2,5-dicarboxybenzamido)                                      13             3-(2-carboxybenzamido)                                          14             3-(2-carboxymethoxyacetamido)                                   15             4-(2-carboxymethoxyacetamido)                                   16             3-(2-carboxymethylthioacetamido)                                17             4-(2-carboxymethylthioacetamido)                                18             3-(3-carboxypropionamido)                                       19             3-(3-carboxyacrylamido)                                         20             4-sulpho                                                        21             4-carboxy                                                       22             3-carboxy                                                       23             3,5-dicarboxy                                                   24             4-(2-carboxyethylene)                                           25             4-(4-carboxytolylamino)                                         ______________________________________                                    

The substituted 1-phenyl-5-mercapto-tetrazole compounds as defined above can be prepared as described in various publications e.g. Research Disclosure 24236 of June 1984, pages 274 to 278.

Preferred substituted 1-phenyl-5-mercapto-tetrazoles for use in accordance with the present invention are 1-phenyl-5-mercaptotetrazoles being substituted by one or two substituents selected from the group consisting of:

(i) a --COOM group with M representing hydrogen, a metal cation, or an ammonium group,

(ii) a --SO₃ M¹ group, M¹ representing hydrogen, a metal cation or an ammonium group,

(iii) an amido group selected from the group consisting of --NR² --CO--R³, --NR² --SO₂ --R³, --NHCO--OR³, --NH--CO--NR² --R³, --NH--OC--NR² --COR³ and --NH--CO--NR² --SO R³, with R² representing a substituent selected from the group consisting of hydrogen and C₁ -C₄ alkyl and R³ representing a substituent selected from the group consisting of C₁ -C₈ alkyl, aryl, and alkenyl wherein said substituent is optionally substituted by a --COOM group and or --SO₃ M¹ group M and M¹ having the same meaning as defined above, and/or said substituent optionally contains an ether, thioether or bivalent amine function.

More preferred substituted 1-phenyl-5-mercapto-tetrazoles for use in accordance with the present invention are 1-phenyl-5-mercapto-tetrazoles being substituted by one or two substituents selected from the group consisting of:

(i) a --COOM group with M representing hydrogen, a metal cation, or an ammonium group,

(ii) a --SO₃ M¹ group, M¹ representing hydrogen, a metal cation, or an ammonium group,

(iii) an amido group selected from the group consisting of --NR² --CO--R⁴ and --NR² --SO₂ --R⁴, with R² having the same meaning as above and R⁴ representing a substituent selected from the group consisting of C₁ -C₈ alkyl, aryl, and alkenyl wherein said substituent is --SO₃ M¹ group, M and M¹ substituted by at least one --COOM group or --SO₃ M¹ group, M and M¹ having the same meaning as defined above, and said substituent optionally contains an ether, thioether or bivalent amine function.

Most preferred substituted 1-phenyl-5-mercapto-tetrazoles according to the present invention are 1-phenyl-5-mercapto-tetrazoles substituted by an amido group selected from the group consisting of --NR² --CO--R⁴ and --NR² --SO₂ --R⁴, with R² and R⁴ having the same meaning as above.

The substituted 1-phenyl-5-mercapto-tetrazole compounds according to the present invention are preferably contained at least partially and more preferably completely in one or more layers of the imaging element. Most preferably the substituted 1-phenyl-5-mercapto-tetrazole compounds according to the present invention are contained in the photosensitive silver halide emulsion containing layer, but they can be contained at least partially in one or more layers in water permeable relationship with said silver halide emulsion containing layer.

Preferably the amount of substituted 1-phenyl-5-mercapto-tetrazole compound in accordance with the present invention in the imaging element is between 0.1 mmole and 10 mmole/mole AgX, more preferably between 0.5 mmole and 5 mmole/mole AgX and most preferably between 1 mmole and 3 mmole/AgX.

However at least part of the substituted 1-phenyl-5-mercapto-tetrazole compounds according to the present invention may be present in the alkaline processing liquid used for developing the image-wise exposed imaging element

The substituted 1-phenyl-5-mercapto-tetrazole compounds in accordance with the present invention are known to improve the density and the tone of positive images, obtained according to the DTR-process as disclosed in EP-A-218752.

However from these disclosure it could not be expected that the printing properties could be improved by the presence of such compounds during the development step of a lithographic printing plate according to the DTR-process.

The imaging element is preferably prepared by coating the different layers on a hydrophilic base. Alternatively the different layers may be laminated to said image receiving layer from a temporary base holding the layers in reverse order as disclosed in U.S. Pat. No. 5,068,165.

The temporary base is preferably a cellulose triacetate or polyethylene terephtalate film base coated with a subbing layer with a weak adhesion for the photosensitive layer or layer packet. Such subbing layer preferably comprises a copolymer containing water-soluble monomers and water-insoluble monomers in a molar ratio between 1:99 and 20:80. An example of an especially preferred copolymer for use in said subbing layer is a copolymer of crotonic acid and vinylacetate in a molar ratio between 2:98and 10.:90.

The hydrophilic base can be a hardened hydrophilic layer, containing a hydrophilic synthetic homopolymer or copolymer and being hardened with a hydrolyzed tetraalkyl orthosilicate crosslinking agent coated on a flexible hydrophobic support. More preferably the hydrophilic base is an aluminum support.

The aluminum support of the imaging element for use in accordance with the present invention can be made of pure aluminum or of an aluminum alloy, the aluminum content of which is at least 95%. The thickness of the support usually ranges from about 0.13 to about 0.50 mm.

The preparation of aluminum or aluminum alloy foils for lithographic offset printing comprises the following steps: graining, anodizing, and optionally sealing of the foil.

Graining and anodization of the foil are necessary to obtain a lithographic printing plate that allows to produce high-quality prints in accordance with the present invention. Sealing is not necessary but may still improve the printing results. Preferably the aluminum foil has a roughness with a CLA value between 0.2 and 1.5 μm, an anodization layer with a thickness between 0.4 and 2.0 μm and is sealed with an aqueous bicarbonate solution.

The graining, anodizing, and sealing of the aluminum foil can be performed as described in e.g. in EP-A 567178, U.S. Pat. No. 3,861,917 and in the documents referred to therein.

To promote the image sharpness and, as a consequence thereof, the sharpness of the final printed copy, the anodization layer may be coloured in the mass with an antihalation dye or pigment e.g. as described in JA-Pu-58-14,797.

The image receiving layer for use in accordance with the present invention is preferably free of hydrophilic binder but may comprise small amounts up to 30% by weight of the total weight of said layer of a hydrophilic colloid e.g. polyvinyl alcohol to improve the hydrophilicity of the layer.

Preferred development nuclei for use in accordance with the present invention are sulphides of heavy metals e.g. sulphides of antimony, bismuth, cadmium, cobalt, lead, nickel, palladium, platinum, silver, and zinc. Other suitable development nuclei are salts such as e.g. selenides, polyselenides, polysulphides, mercaptans, and tin (II) halides. Heavy metals, preferably silver, gold, platinum, palladium, and mercury can be used in colloidal form. More preferred development nuclei for use in accordance with the present invention are nuclei, especially sulphides of heavy metals having an average diameter less than 6 nm and wherein the number of nuclei having a diameter larger than 4.5 nm is less than 15% of the total number of nuclei contained in said image receiving layer as disclosed in EP-A 546598. Especially preferred development nuclei in connection with the present invention are palladium sulphide nuclei having an average diameter less than 6nm and wherein the number of nuclei having a diameter larger than 4.5 nm is less than 15% of the total number of nuclei contained in said image receiving layer.

To promote the image sharpness and, as a consequence thereof, the sharpness of the final printed copy, the hydrophilic base can be provided with a very thin antihalation coating of a dye or pigment, which may be applied before or after said image receiving layer is applied. Also to promote the image sharpness the image receiving layer may incorporate at least one antihalation dye or pigment. The usual dyes and pigments can be chosen such that they prevent or reduce halation in the silver halide emulsions having any desired photosensitivity range comprised between 300 and 900 nm.

The photosensitive layer used in accordance with the present invention may be any silver halide emulsion layer being in water permeable relationship with said image receiving layer, comprising a hydrophilic colloid binder, at least one of the silver halide emulsions being photosensitive.

Layers being in waterpermeable contact with each other are layers that are contiguous to each other or only separated from each other by (a) waterpermeable layer(s). The nature of a waterpermeable layer is such that it does not substantially inhibit or restrain the diffusion of water or of compounds contained in an aqueous solution e.g. developing agents or complexed silver ions.

The photographic silver halide emulsion(s) for coating silver halide emulsion layers in accordance with the present invention can be prepared from soluble silver salts and soluble halides according to different methods as described e.g. by P. Glafkides in "Chimie et Physique Photographique" Paul Montel, Paris (1967), by G. F. Duffin in "Photographic Emulsion Chemistry", The Focal Press, London (1966), and by V. L. Zelikman et al in "Making and Coating Photographic Emulsion", The Focal Press, London (1966).

According to the present invention the silver halide emulsion or emulsions preferably consist principally of silver chloride while a fraction of silver bromide may be present ranging from 1 mole % to 40 mole %. The silver halide emulsions preferably belong to the core/shell type well known to those skilled in the art in the sense that substantially all the bromide is concentrated in the core. This core contains preferably 10 to 40 % of the total silver halide precipitated, while the shell consists preferably of 60 to 90 % of the total silver halide precipitated. Most preferably a silver halide emulsion containing at least 70 mole % of silver chloride is used.

The average size of the silver halide grains may range from 0.10 to 0.70 μm , preferably from 0.25 to 0.45 μm.

Preferably during the precipitation stage iridium and/or rhodium containing compounds or a mixture of both are added. The concentration of these added compounds ranges from 10⁻⁸ to 10⁻³ mole per mole of AgNO₃, preferably between 10⁻⁷ and 10⁻⁶ mole per mole of AgNO₃.

The silver halide emulsions can be chemically sensitized. A method of chemical sensitization has been described in the article of R. KOSLOWSKY, Z. Wiss. Photogr. Photophys. Photochem. 46, 65-72 (1951).

The silver halide emulsions of the DTR-element can be spectrally sensitized according to the spectral emission of the exposure source for which the DTR element is designed.

Suitable sensitizing dyes for the visible spectral region include methine dyes such as those described by F. M. Hamer in "The Cyanine Dyes and Related Compounds", 1964, John Wiley & Sons.

The spectral photosensitivity of the silver halide can also be adjusted for exposure by laser light e.g. helium-neon laser light, argon laser light, and solid state laser light. Dyes that can be used for adjusting the photosensitivity to laser light have been disclosed i.a. in the documents cited in EP-A-93200339.5 and U.S. Pat. No. 5,200,294.

The silver halide emulsions may contain the usual emulsion stabilizers. Suitable emulsion stabilizers are azaindenes, preferably tetra- or penta-azaindenes, especially those substituted with hydroxy or amino groups. Compounds of this kind have been described by BIRR in Z. Wiss. Photogr. Photophys. Photochem. 47, 2-27 (1952). Other suitable emulsion stabilizers are i.a. heterocyclic mercapto compounds. Especially good results are obtained with the substituted 1-phenyl 5-mercapto-tetrazole compounds in accordance with the present invention, alone or in combination with other emulsion stabilizers.

As binder in the silver halide emulsion layer(s) in connection with the present invention a hydrophilic colloid may be used, usually a protein, preferably gelatin. Gelatin can, however, be replaced in part or integrally by synthetic, semi-synthetic, or natural polymers.

The silver halide emulsions may contain pH controlling ingredients. Preferably the silver halide emulsion layer is coated at a pH value below the isoelectric point of the gelatin to improve the stability characteristics of the coated layer. Other ingredients such as antifogging agents, development accelerators, wetting agents, and hardening agents for gelatin may be present. The silver halide emulsion layer may comprise light-screening dyes that absorb scattering light and thus promote the image sharpness. Suitable light-absorbing dyes are described in i.a. U.S. Pat. No. 4,092,168, U.S. Pat. No. 4,311,787, DE-P-2,453,217.

More details about the composition, preparation and coating of silver halide emulsions can be found in e.g. Product Licensing Index, Vol. 92, December 1971, publication 9232, p. 107-109.

The imaging element preferably also comprises an intermediate layer between the image receiving layer on the hydrophilic base and the photosensitive layer(packet) to facilate the removal of said layer(packet) thereby uncovering the silver image formed in the image receiving layer by processing the imaging element.

In one embodiment, the intermediate layer can be a water-swellable intermediate layer coated at a ratio of 0.01 to 2g/m2 and comprising at least one non-proteinic hydrophilic film-forming polymer and optionally comprising an antihalation dye or pigment as disclosed in EP-A-410500.

In another embodiment, the intermediate layer can be a layer comprising hydrophobic polymer beads having an average diameter not lower than 0.2 μm and having been prepared by polymerization of at least one ethylenically unsaturated monomer. Preferably, said intermediate layer in dry condition comprises said hydrophobic polymer beads in an amount of up to 80% of its total weight. Further details are disclosed in EP-A-483415.

A supplemental intermediate layer, which may be present between said silver halide emulsion containing layer and said water-swellable intermediate layer or said intermediate layer comprising hydrophobic polymer beads may incorporate one or more ingredients such as i.a. antihalation dyes or pigment, developing agents, silver halide solvents, base precursors, and anticorrosion substances.

When the imaging element is prepared by laminating a photosensitive layer or layer packet onto the image receiving layer the intermediate layer(s) are contained in the photosensitive layer packet, the water-swellable intermediate layer or the intermediate layer comprising hydrophobic polymer beads having an average diameter not lower than 0.2 μm and having been prepared by polymerization of at least one ethylenically unsaturated monomer being the upper layer.

A wide choice of cameras for exposing the photosensitive silver halide emulsion exists on the market. Horizontal, vertical and darkroom type cameras and contact-exposure apparatus are available to suit any particular class of reprographic work. The imaging element in accordance with the present invention can also be exposed with the aid of i.a. laser recorders and cathode rays tubes.

The development and diffusion transfer are effected with the aid of an aqueous alkaline solution in the presence of (a) developing agent(s) and (a) silver halide solvent(s). The developing agent(s) and/or the silver halide solvent(s) can be incorporated in the aqueous alkaline solution and/or in said silver halide emulsion layer and/or in any of said intermediate layers and/or in a supplemental hydrophilic colloid layer in water-permeable relationship with said silver halide emulsion layer. The latter supplemental hydrophilic colloid layer can be coated on top of said silver halide emulsion layer remotest from said hydrophilic base.

The silver halide solvent can also be incorporated at least in part in the physical development nuclei containing layer. When the aqueous alkaline solution does not comprise the developing agent(s), it is merely an activating liquid that is capable of dissolving the developing agent(s) contained in one of the layers.

Silver halide developing agents for use in accordance with the present invention are preferably of the p-dihydroxybenzene type, e.g. hydroquinone, methylhydroquinone or chlorohydroquinone, preferably in combination with an auxiliary developing agent being a 1-phenyl-3-pyrazolidinone-type developing agent and/or p-monomethylaminophenol. More details are disclosed in EP-A-93201649.6. However other developing agents can be used. Preferred amounts of the hydroquinone-type developing agents are in the range of 0.05 mole to 0.25 mole per liter and preferred amounts of secondary developing agent(s) in the range of 1.8× 10⁻³ to 2.0×10⁻² mole per liter.

The silver halide solvent, which acts as a complexing agent for silver halide, preferably is a water-soluble thiosulphate or thiocyanate e.g. sodium, potassium, or ammonium thiosulphate and sodium, potassium, or ammonium thiocyanate.

Further silver halide solvents that can be used in connection with the present invention are alkanolamines. Alkanolamines that are suitable for use in accordance with the present invention may be of the tertiary, secondary or primary type. Examples of alkanolamines that may be used in connection with the present invention are disclosed in e.g. EP-A 549830.

According to the present invention the alkanolamines are preferably present in the alkaline processing liquid in a concentration preferably between 0.05% and 10% by weight. However part or all of the alkanolamine can be present in one or more layers of the imaging element.

Further suitable silver halide solvents are thioethers. Preferably used thioethers are disclosed in e.g. U.S. Pat. No. 4,960,683 and U.S. Pat. No. 5,200,294.

Still further suitable silver halide solvents are meso-ionic compounds. Preferred meso-ionic compounds for use in accordance with the present invention are triazolium thiolates and more preferably 1,2,4-triazolium-3-thiolates. More details are disclosed in EP-A 554585.

Still further suitable silver halide solvents are sulphite, amines, 2-mercaptobenzoic acid and those disclosed in "The Theory of the Photographic Process" 4th Ed., edited by T. H. James, pages 474-475. Further interesting silver halide solvents have been described in i.a. U.S. Pat. Nos. 2,857,276, 4,355,090, 4,297,429 and 4,297,430. Among these are cyclic imide compounds such as e.g. uracil and 5,5-dialkylhydantoins. Other suitable silver halide solvents are the alkyl sulfones.

Combinations of different silver halide solvents can be used and it is also possible to incorporate at least one silver halide solvent into a suitable layer of the imaging element and to add at least one other silver halide solvent to the developing solution.

Examples of suitable combinations of different silver halide solvents are a combination of (an) alkanolamine(s) and a thiosulfate as disclosed in EP-A 549831 and 4,6-dihydroxypyrimidines in combination with other silver halide solvents as disclosed in EP-A 549830.

The aqueous alkaline solution may further comprise sulphite e.g. sodium sulphite in an amount ranging from 40 g to 180 g per liter, preferably from 60 to 160 g per liter, and a silver halide solvent, preferably a water soluble thiosulphate and/or thiocyanate in an amount ranging from 5 g to 20 g per liter.

Combinations of at least one silver halide solvent and a regulator may also be used. Suitable regulators are disclosed in EP-A 547660 and in EP-A 576736.

The aqueous alkaline solution used according to the present invention preferably comprises aluminum ions in an amount of at least 0.3 g/1 in order to prevent sticking of the emulsion layer to the transporting rollers when the emulsion is swollen with the aqueous alkaline solution. More preferably the aqueous alkaline solution used in accordance with the present invention comprises aluminum ions in an amount of at least 0.6 g/1.

The alkaline processing liquid preferably has a pH between 9 and 14 and more preferably between 10 and 13. Said pH may be established by an organic or inorganic alkaline substance or a combination thereof. Suitable inorganic alkaline substances are e.g. potassium or sodium hydroxide, carbonate, phosphate etc. Suitable organic alkaline substances are e.g. alkanolamines. In the latter case the alkanolamines will provide or help providing the pH and serve as a silver halide complexing agent.

The processing conditions such as pH, temperature and time may vary within broad ranges provided the mechanical strength of the materials to be processed is not adversely influenced and no decomposition takes place.

The aqueous alkaline solution may further comprise hydrophobizing agents for improving the hydrophobicity of the silver image obtained in the image image receiving layer. Generally these compounds contain a mercapto group or thiolate group and one or more hydrophobic substituents. Examples of hydrophobizing agents are e.g. those described in U.S. Pat. No. 3,776,728, and U.S. Pat. No. 4,563,410. Particularly preferred hydrophobizing agents are long chain (at least 5 carbon atoms) alkyl substituted mercaptotetrazoles. More details are disclosed in EP-A-93201649.6. The hydrophobizing agents can be used alone or in combination with each other.

The aqueous alkaline solution may comprise other ingredients such as e.g. oxidation preservatives, a compound releasing bromide ions, calcium-sequestering compounds, anti-sludge agents, and hardeners including latent hardeners.

Regeneration of the aqueous alkaline solution according to known methods is, of course, possible, whether the solution incorporates developing agent(s) and/or silver halide solvent(s) or not.

The development may be stopped--though this is often not necessary--with a so-called stabilization liquid, which actually is an acidic stop-bath having a pH preferably in the range of 5 to 6 as disclosed in EP-A-519,123.

The development and diffusion transfer can be initiated in different ways e.g. by rubbing with a roller, by wiping with an absorbent means e.g. with a plug of cotton or sponge, or by dipping the material to be treated in the liquid composition. Preferably, they proceed in an automatically operated apparatus. They are normally carried out at a temperature in the range of 18° C. to 30° C.

After formation of the silver image on the lithographic image receiving element an excess of alkaline solution still present on the monosheet layer assemblage may be eliminated, preferably by guiding the monosheet layer assemblage through a pair of squeezing rollers.

The silver image thus obtained in the image receiving layer is subsequently uncovered by treating the imaging element to remove the photosensitive layer and the intermediate layer.

Various embodiments for removing the photosensitive layer and the intermediate layer(s) are disclosed in EP-A 483,415

According to a particularly preferred embodiment for removing the photosensitive layer(s) and the intermediate layer(s) the imaging element is held under a spray or jet of rinsing water or rinsing aqueous medium. The rinsing aqueous medium used to detach the intermediate layer(s) and the emulsion layer(s) (s) by rinsing may comprise ingredients such as i.a. weak acidifying agents, wetting agents, and hardeners including latent hardeners.

The temperature of the rinsing water may be varied widely but is preferably between 20° C. and 30° C.

The imaged surface of the lithographic base can be subjected to a chemical treatment that increases the hydrophilicity of the non-silver image parts and the oleophilicity of the silver image.

This chemical after-treatment is preferably carried out with a lithographic composition often called fixer, which comprises at least one compound enhancing the ink-receptivity and/or lacquer-receptivity of the silver image, and also comprises at least one compound that improves the ink-repelling characteristics of the hydrophilic grained and anodized aluminum support. More details are disclosed in EP-A-93201878.1.

At the moment the treatment with the fixer is started the surface carrying the silver pattern may be in dry or wet state. In general, the treatment with the fixer does not take long, usually not longer than about 30 seconds and it may be carried out immediately after the processing and uncovering steps.

The fixer can be applied in different ways such as by rubbing with a roller, by wiping with an absorbent means e.g. with a plug of cotton or sponge, or by dipping the material to be treated in the fixer. The image-hydrophobizing step of the printing plate may also proceed automatically by conducting the printing plate through a device having a narrow channel filled with the fixer-and conveying the printing plate at the end of the channel between two squeezing rollers removing the excess of liquid.

The following examples illustrate the present invention without however, limiting it thereto. All parts, percentages and ratios are by weight unless otherwise indicated.

EXAMPLE 1 (Comparative example)

An imaging element I was obtained by coating a grained, anodized and sealed aluminum support with a silver-receptive stratum containing 0.7 mg/m² PdS as physical development nuclei.

An intermediate layer was then provided on the dry silver-receptive stratum from an aqueous composition in such a way that the resulting dried layer had a weight of 0.5 .g of polymethyl methacrylate beads per m², said composition comprising:

    ______________________________________                                         a 20% dispersion of polymethyl methacrylate beads                                                          50     ml                                          in a mixture of equal volumes of water and ethanol                             having an average diameter of 1.0 μm                                        Helioechtpapierrot BL (trade mark for a dye sold by                                                        2.5    g                                           BAYER AG, D-5090 Leverkusen, West-Germany)                                     saponine                    2.5    g                                           sodium oleylmethyltauride   1.25   g                                           demineralized water         300    ml                                          (pH-value: 5.6)                                                                ______________________________________                                    

Finally a substantially unhardened photosensitive negative-working cadmium-free gelatin silver chlorobromoiodide emulsion layer (97.98/2/0.02 mol%) containing 1 mmole/mole AgX of compound A (4-hydroxy-6-methyl-1,3,3a,7-tetraazaindene) was coated on the intermediate layer, the silver halide being provided in an amount corresponding to 2.40 g of silver nitrate per m² and the gelatin content of the emulsion layer being 1.58 g/m².

The imaging elements II, III, IV were prepared in an identical way except that compound A was substituted respectively by compounds B [7-sulpho-naphto-(2,3-d)-oxazoline], C (1-phenyl-5-mercapto-tetrazole) and compound 10 (Cpd 10) of Table 1.

Those 4 imaging elements were identically exposed through a contact screen in a process-camera and immersed for 8 s. at 24° C. in a freshly made developing solution having the following ingredients:

    ______________________________________                                         carboxymethylcellulose    4       g                                            sodium hydroxide          22.5    g                                            anhydrous sodium sulphite 120     g                                            hydroquinone              20      g                                            1-phenyl-3-pyrazolidinone 6       g                                            potassium bromide         0.75    g                                            anhydrous sodium thiosulphate                                                                            8       g                                            ethylene diamine tetraacetic acid tetrasodium salt                                                       2       g                                            demineralized water to make                                                                              1000    ml                                           pH (24° C.) = 13                                                        ______________________________________                                    

The initiated diffusion transfer was allowed to continue for 30 s to form a silver image in the image receiving layers.

To remove the developed silver halide emulsion layer and the intermediate layer from the aluminum foils the developed monosheet DTR materials were rinsed for 30 s with a water jet. at 20° C.

Next, the imaged surface of the aluminum foil was rubbed with a fixer to enhance the water-receptivity of the non-image areas and to make the image areas oleophilic ink-receptive. The fixer had the following composition:

    ______________________________________                                         10% aqueous n-hexadecyl trimethyl ammonium                                                                 25     ml                                          chloride                                                                       20% aqueous solution of polystyrene sulphonic acid                                                         100    ml                                          potassium nitrate           12.5   g                                           citric acid                 20.0   g                                           1-phenyl-5-mercaptotetrazole                                                                               2.0    g                                           sodium hydroxide            5.5    g                                           water to make               1000   ml                                          pH (20° C.) = 4                                                         ______________________________________                                    

The amount of silver deposited (silver yield) in the image receiving layer was then measured using an analytical X-ray Fluorescence Spectrophotometer PHILIPS 1400 (tradename of Philips-The Netherlands). The results obtained for each of the 4 materials are given in table 2.

                  TABLE 2                                                          ______________________________________                                         Sample     Compound added                                                                              Ag yield (g/m.sup.2)                                   ______________________________________                                         I          A            0.98                                                   II         B            1.04                                                   III        C            1.02                                                   IV         Cpd 10 of table 1                                                                           1.10                                                   ______________________________________                                    

Evaluation:

From the above it can be seen that with Compound 10 of Table 1 the silver yield is improved. Since the silver yield is an important parameter influencing the printing endurance of the printing plate the printing endurance can be increased with a substituted 1-phenyl-5-mercapto-tetrazole as defined in the statement of the invention.

EXAMPLE 2 (Comparative example)

The imaging elements III and IV were prepared according to Example 1. The imaging elements V and VI were prepared in an identical way except that compound A was substituted by compound 16 (Cpd 16), respectively compound 21 (Cpd 21) of Table 1. After exposing, developing, rinsing and fixing the 4 elements, the amount of silver deposited (silver yield) in the image receiving layer was then measured using an analytical X-ray Fluorescence Spectrophotometer PHILIPS 1400 (tradename). The results obtained for each of the 4 materials are given in Table 3.

                  TABLE 3                                                          ______________________________________                                         Sample     Compound added                                                                              Ag yield (g/m.sup.2)                                   ______________________________________                                         III        C            1.01                                                   IV         Cpd 10 of table 1                                                                           1.10                                                   V          Cpd 16 of table 1                                                                           1.20                                                   VI         Cpd 21 of table 1                                                                           1.08                                                   ______________________________________                                    

Evaluation:

From the above it can be seen that with the substituted 1-phenyl-5-mercapto-tetrazoles substituted according to the invention the silver yield is improved. Since the silver yield is an important parameter influencing the printing endurance of the printing plate the printing endurance can be increased with said 1-phenyl-5-mercapto-tetrazoles.

EXAMPLE 3 (Comparative example)

The imaging elements I, II, IV were prepared according to example 1. The imaging element VII was prepared in an identical way except that compound A was substituted by compound E (benzotriazole). After exposing, developing, rinsing and fixing the 4 elements, the amount of silver deposited (silver yield) in the image receiving layer was then measured using an analytical X-ray Fluorescence Spectrophotometer PHILIPS 1400 (tradename). The results obtained for each of the 4 materials are given in Table 4.

The printing plates thus prepared were mounted on the same offset printing machine (Heidelberg GTO-46) and were printed under identical conditions. A commercial dampening solution AQUA TAME (tradename) was used at a 5% concentration as fountain solution, and K+E 125 as ink. A compressible rubber blanket was used.

The results are given in the following Table 4.

The lithographic properties such as ink acceptance and printing contrast were evaluated as follows:

a) ink acceptance: the number of copies that has to be printed before they are free of any white Spot in the ink accepting areas:

b) printing contrast: the number of steps of a wedge with a wedge constant of 0.10 log E that can be discerned on the 25th copy.

                  TABLE 4                                                          ______________________________________                                                             Ag yield Ink      Printing                                 Sample                                                                               Added compound                                                                               (g/m.sup.2)                                                                             acceptance                                                                              contrast                                 ______________________________________                                         I     A             1.04     20       7                                        II    B             1.01     20       7                                        IV    Cpd 10 of table 1                                                                            1.12     15       5                                        VII   E             1.10     20       7                                        ______________________________________                                    

Evaluation:

From the above it can be seen that when a DTR imaging element is used comprising a 1-phenyl-5-mercapto-tetrazoles substituted as defined herein, printing plates with a good ink acceptance and a high printing contrast are obtained. 

We claim:
 1. A method for making an offset printing plate according to the silver salt diffusion transfer process comprising the steps of:(a) image-wise exposing an imaging element comprising in the order given (i) a hydrophilic base, (ii) an image receiving layer containing physical development nuclei and (iii) a photosensitive layer containing a silver halide emulsion being in water permeable relationship with said image receiving layer, (b) applying an aqueous alkaline solution to the imaging element in the presence of (a) developing agent(s) and (a) silver halide solvent(s) to form a silver image in said photosensitive layer and to allow unreduced silver halide or complexes formed thereof to diffuse image-wise from the photosensitive layer to said image receiving layer to produce therein a silver image, (c) treating the imaging element to remove the layer(s) on top of the image receiving layer, thereby uncovering said silver image formed in said image receiving layer,characterized in that the development step is carried out in the presence of a 1-phenyl 5-mercapto-tetrazole which is substituted by an amido group selected from the group consisting of --NR¹ --CO--R² and --NR¹ --SO₂ --R², with R¹ representing a substituent selected from the group consisting of hydrogen and C₁ -C₄ alkyl and R² representing a substituent selected from the group consisting of C₁ -C₈ alkyl, aryl, and alkenyl wherein said substituent is substituted by at least one --COOM group or --SO₃ M¹ group with M representing hydrogen, a metal cation, or an ammonium group and M¹ representing hydrogen, a metal cation, or an ammonium group and said substituent optionally contains an ether, thioether or bivalent amine function.
 2. A method according to claim 1 wherein said hydrophilic base is a grained and anodized aluminum support.
 3. An imaging element comprising in the order given (i) a hydrophilic base, (ii) an image receiving layer containing physical development nuclei and (iii) a photosensitive layer containing a silver halide emulsion being in water permeable relationship with said image receiving layer, characterized in that said photosensitive layer comprises a 1-phenyl 5-mercapto-tetrazole, being substituted by an amido group selected from the group consisting of --NR¹ --CO--R² and --NR¹ --SO₂ --R² with R¹ representing a substituent selected from the group consisting of hydrogen and C₁ -C₄ alkyl and R² representing a substituent selected from the group consisting of C₁ -C₈ alkyl, aryl, and alkenyl wherein said substituent is substituted by at least one --COOM group or --SO₃ M¹ group, with M representing hydrogen, a metal cation, or an ammonium group and M¹ representing hydrogen, a metal cation, or an ammonium group and said substituent optionally contains an ether, thioether or bivalent amine function.
 4. An imaging element according to claim 3 wherein the amount of said 1-phenyl 5-mercapto-tetrazole compound in said photosensitive layer is between 0.1 mmole and 10 mmole/mole AqX. 